A general configuration of a pulse radar is illustrated in FIG. 6. In the pulse radar, a magnetron 1 outputs a pulse signal having an oscillation frequency of, for example, 9.41 GHz (e.g., a pulse width of 1 μs, an output of 10 kW), the pulse signal is emitted into the air from an antenna 3 via a circulator 2, a signal reflected from an object is received by the antenna 3 again, and the signal is input to a limiter 4. The signal which is input via the limiter 4 to a frequency converter 5 is mixed with an output (local oscillation signal of, for example, 9.47 GHz) of a local oscillator 6, to be converted into an intermediate frequency (IF signal) of, for example, 60 MHz. The converted IF signal is amplified by an IF amplifier 7, is converted into a video signal by a signal processing circuit 8, and an image is displayed on a radar display device (PPI). Note that the limiter 4 is provided so as to prevent a large signal from being input to break the frequency converter 5. In recent years, a microwave integrated circuit (MIC) is used to integrate the limiter 4 with the frequency converter 5.
A conventional circuit configuration of a microwave frequency converter for use in a pulse radar as illustrated in FIG. 6, is illustrated in FIG. 7. In the conventional microwave frequency converter, a pulse-modulated microwave signal (RF signal) input through an input terminal 11 of an MIC limiter 10 is amplified by an RF amplifier 12, the resultant signal is input to a double balanced mixer 13 to be mixed with a signal (local oscillation signal) of a local oscillator 14, two IF outputs are combined by an IF output combiner 15, and the combined output is obtained at an IF output terminal 16 (see Patent Document 1).
Patent Document 1: JP 2001-111447 A
The radar pulse signal emitted from the antenna is received as a smaller signal as an object reflecting the signal is more distant, and is received as a larger signal as an object reflecting the signal is nearer, or is received as a smaller signal as an object reflecting the signal is smaller, and is received as a larger signal as an object reflecting the signal is larger. The above-described conventional microwave frequency converter has the following drawbacks. FIG. 8(A) illustrates an IF output power with respect to an RF input power of a frequency converter with RF amplifier. FIG. 8(B) illustrates an IF output power with respect to an RF input power of a frequency converter without RF amplifier. Specifically, the frequency converter with RF amplifier of FIG. 8(A) amplifies a signal using the RF amplifier, and therefore, is suitable for detection of a distant or small reflective object. However, as illustrated in FIG. 8(A), saturation starts when the RF signal is at a level of, for example, −5 dBm, so that a signal of −5 dBm or more which is reflected from a near or large reflective object cannot be detected due to saturation. In other words, a signal from a short distance cannot be received.
In contrast to this, the frequency converter without RF amplifier can detect an input of up to +3 dBm without saturation as illustrated in FIG. 8(B).
Note that the frequency converter without RF amplifier is not suitable for detection of a distant or small reflective object, since the frequency converter without RF amplifier does not amplify a signal.
In recent years, however, to avoid collision between ships or between a ship and a fixed object is becoming the main purpose of radars, and performance capable of detecting a nearer reflective object than the status quo is desired. Performance of detecting a distant or small reflective object, which is conventionally possessed by radars, is indispensable. In other words, a radar capable of receiving a signal within a range of a long distance to a short distance (shorter than that of conventional products) is becoming desired.
In the pulse radar, the magnetron 1 outputs a pulse signal having an oscillation frequency of, for example, 9.41 GHz (e.g., a pulse width of 1 μs, an output of 10 kW), so that an excessively large power which leaks directly to the limiter 4, though the circulator 2 is provided therebetween, is input to the frequency converter 5.
Due to the excessively large power, the oscillation frequency of the local oscillator 6 used in the frequency converter 5 is changed. Therefore, an intermediate frequency (IF signal) of, for example, 60 MHz is changed. When the intermediate frequency (IF signal) is changed, the amplification degree of an intermediate frequency amplifier is changed, and in an extreme case, the reception sensitivity is reduced. Therefore, it is desired that the oscillation frequency of the local oscillator 6 be not changed.
FIG. 9 illustrates a conventional microwave frequency converter which have both characteristics of FIGS. 8(A) and (B).
Specifically, in the conventional microwave frequency converter, a PIN switch 17 is provided before the RF amplifier 12. The PIN switch 17 switches ON/OFF an RF input based on a trigger input.
Before an excessively large RF signal directly input from the magnetron of the pulse radar is applied to the RF amplifier 12, the PIN switch 17 is switched ON with a trigger input. The ON state is held until the end of the input of the excessively large reflected signal from a short distance to the RF amplifier 12. When the RF signal input to the RF amplifier 12 becomes less than or equal to a saturated input of the RF amplifier 12, the PIN switch 17 is switched OFF, thereby returning to the ordinary microwave frequency converter.
However, even when the PIN switch 17 is in the OFF state, an insertion loss of about 1 dB remains. Therefore, the microwave frequency converter with the PIN switch 17 is not suitable for detection of a distant or small reflective object, compared to the conventional microwave frequency converter without the PIN switch 17. Also, the PIN switch 17 composed of a single stage of PIN diode has an attenuation amount of about 15 dB, and the attenuation amount is insufficient for an excessively large reflected signal from a considerably short distance. When two stages of PIN diodes are provided so as to increase the attenuation amount, the residual insertion loss disadvantageously increases.
To solve the above-described conventional problems, the present invention provides a microwave frequency converter which has the same performance as that of conventional microwave frequency converters with RF amplifier and can receive a reflection from a distant or small object, and can receive a signal from a considerably shorter distance than when a PIN switch is switched ON/OFF, and in which the oscillation frequency of the local oscillator 6 is not changed due to a direct power from the magnetron 1.